As is well known, potential electrical hazards exist in practically all patient care environments. Electrically controlled beds and even innocuous devices such as bedside lamps, electric heaters, call buttons and the like are sources of potentially lethal electrical current. Furthermore, high current electrical shock (Macroshock) applied through intact skin, such as through the hand when grasping or touching an unsafe electrical device, constitutes a constant potential danger in the event there is inadequate grounding. Inadequate grounding might be the result of poor equipment design, a broken ground wire in the power cord or power plug, or simply the result of an improperly wired wall outlet.
If an electrical hospital bed is properly designed and connected to a properly wired wall outlet, leakage currents will be conducted harmlessly to ground. Normally, the bed frame is connected through the third wire in the power plug to the ground of the electrical system. Such a ground connection is a safety feature to prevent accidental shock to patients or medical personnel.
However, in the event of an internal circuit defect in the components of the hospital bed, voltage may appear on the metal bed frame and will attempt to ground through the third wire. If the ground wire is broken or disconnected either accidentally or intentionally, the voltage can no longer return to ground, resulting in a troublesome situation. The voltage will now seek a return path to ground through the body of any person in contact with the bed frame or instrument. This situation is particularly dangerous if the person is also in contact or touching any grounded metal surface as a water pipe, radiator or other instrument, or is standing on a damp floor because the resultant shock may cause great injury and even death.
The use of isolation transformers has been suggested as a method of reducing the electrical shock hazard. Such a transformer allows connection of electronic instruments or different type of machines, motors, etc. to the electrical power service through a non-metallic path way. Although isolation transformers do in fact reduce macroshock hazards, they have certain disadvantages such as great expense, bulk and the fact that that they do not eliminate the possibility of leakage current in the microshock range.
Moreover, the critical importance of maintaining the integrity of the ground connection between the frame of the hospital bed and the electrical power supply source has led to the development of various expensive protective systems designed to test for an open circuit in the ground connection which do not really overcome the basic problems.
The electrical systems which have been employed heretofore to solve the problems outlined above, have not been fully effective in obtaining a continuous monitoring of the integrity of the ground connection for the frame. They also have limitations on their reliability in immediately detecting an open ground connection and cause the power to be interrupted without endangering medical personnel or patients to electric shock.
Various devices and circuits for the detection and elimination of ground faults have been proposed, some sensing the effective impedance of the system ground and others operating on an unbalance which ground currents may produce. The disadvantage of such detectors lies not only in their unreliability but also in the sensitivity required for detection of small leakage currents which makes them unstable at times, whereby indications of fault occur even under normal operating conditions when no fault exists.